Conventionally, as a high withstand voltage semiconductor device, there has been the semiconductor device described in JP-A-2006-148058 corresponding to U.S. Pat. No. 7,239,181. FIG. 10 is a diagram of an equivalent circuit of the semiconductor device 1000 in JP-A-2006-148058.
As shown in FIG. 10, in the semiconductor device 1000, n (n≧2) transistor elements Tr1 to Trn that are insulated and separated from each other are sequentially series-connected between a ground (GND) potential and a predetermined potential Vs, with the transistor element near the GND potential being a first stage and the transistor element near the predetermined potential Vs being an n-th stage. Further, the gate terminal of the first stage transistor element Trn serves as an input terminal of the semiconductor device 1000, and n resistive elements R1 to Rn are sequentially series-connected between the GND potential and the predetermined potential Vs, with the resistive element near the GND potential being a first stage and the resistive element near the predetermined potential Vs being an n-th stage. Additionally, the gate terminals of the transistor elements Tr2 to Trn of each stage excluding the first stage transistor element Tr1 are sequentially connected to connection points between the series-connected resistive elements R1 to Rn of each stage, and an output is removed via a load resistor (not shown) having a predetermined resistance value from the terminal of the transistor element Trn of the n-th stage near the predetermined potential Vs.
The n transistor elements Tr1 to Trn in this semiconductor device 1000 are formed on an n-conductive type semiconductor layer of an SOI structure semiconductor substrate including an embedded oxide film. Further, the n transistor elements Tr1 to Trn comprise N channel LDMOS or the like, for example, and are insulated and separated from each other by element separating trenches reaching the embedded oxide film.
Additionally, multiple field separating trenches reaching the embedded oxide film are formed, and the n transistor elements Tr1 to Trn that are insulated and separated from each other are sequentially disposed one at a time so as to include the high stage transistor element in each field region surrounded by the field separating trenches.
Thus, the voltage applied to each field region surrounded by the field separating trenches can be equalized in accordance with a voltage increase from the GND potential to the predetermined potential Vs, and the voltage range handled by the n transistor elements Tr1 to Trn can be moved in order from the GND potential to the predetermined potential Vs. Consequently, even with transistor elements having a normal withstand voltage that can be fabricated inexpensively using a common fabricating method, there can be configured a semiconductor device that ensures a high withstand voltage required overall by appropriately setting the number n of the transistor elements.
However, in the semiconductor device 1000 described in JP-A-2006-148058, the elements (transistor elements, resistive elements, etc.) are electrically interconnected by aluminum wiring. Consequently, there has been the problem that when testing of the semiconductor device 1000 is to be implemented, defects when testing is performed by a combination of the elements can be detected, but testing of element units (e.g., minute resistance value shifts, etc.) cannot be performed. Consequently, there is the potential for durability deterioration of element units and characteristic fluctuations to occur.
Thus, it is required for a semiconductor device with high reliability.
Further, in a power device that drives a three-phase motor, for example, the main power supply has a high voltage of 100 V to 400 V, for example. For this reason, a high-voltage IC (HVIC) disposed with a photocoupler and an LDMOS transistor is used for the drive circuit that drives the power device.
In high-voltage ICs that handle a high voltage of 600 V or greater, for example, a withstand voltage of 600 V or greater becomes necessary, but because it is difficult to ensure a withstand voltage of 600 V or greater with a stand-alone LDMOS transistor, a tandem structure is known which ensures a withstand voltage by connecting LDMOS transistors having a withstand voltage of 600 V or less.
However, in this tandem structure, the voltage dividing resistance value that voltage-divides the LDMOS transistors is several MΩ, which is high. For this reason, there has been the problem that the current ends up flowing to the parasitic capacitance on the substrate side when there is a surge in the high-voltage IC, the voltage applied to one of the LDMOS transistors becomes larger, and the withstand current rating resultantly becomes smaller.
Thus, a structure has been proposed in JP-A-2006-148058, for example, which disperses the voltage of each stage of the tandem structure to prevent surge breakdown by disposing smoothing capacitors in parallel with voltage dividing resistors and allowing a surge current to flow.
In a high-voltage IC having the above-described structure, it is conceivable, when testing the leak current of the smoothing capacitor and the resistance value of the voltage dividing resistor of each stage in the tandem structure, to dispose test pads on both ends of these smoothing capacitors and voltage dividing resistors and allow a current to flow to each pad to detect the leak current of the smoothing capacitors.
However, because the smoothing capacitors is connected in parallel with the voltage dividing resistors, the current flowing to each of the test pads ends up flowing to the voltage dividing resistors. For this reason, even when an extremely weak leak current occurs in the smoothing capacitor, that abnormality has not been able to be detected.
Thus, it is required for a semiconductor device to test the leak current of a smoothing capacitor and the resistance value of a voltage dividing resistor correctly.